1. Field of the Invention
The invention relates to a catalyst system, and more particularly to a hydrogenation catalyst system and a selective hydrogenation process utilizing the same.
2. Description of the Related Art
A polymer of conjugated diene monomer such as 1,3-butadiene or isoprene, or a copolymer with a vinyl aromatic monomer such as styrene that may be copolymerized with the conjugated diene monomer, has been widely used as an elastomer in many fields. However, the weather resistance, thermal resistance, oxidation resistance and ozone resistance of such copolymers are apparently insufficient with the unsaturated carbon-carbon double bonds within the main chains, limiting the use range of the polymers.
In an effort to improve the durability and oxidation resistance of a polymer having unsaturated double bonds, a process for selectively hydrogenating the double bonds in the polymers may be generally adopted.
Various methods to hydrogenate polymers having olefinic unsaturated double bonds have been reported, and could be classified into two main catalogs. The first one is to use a heterogeneous catalyst, while the second one is to use a Ziegler catalyst or a homogeneous catalyst belonging to organometallic compounds such as cobalt or nickel or titanium.
A typical heterogeneous catalyst consists of a catalytically active ingredient and a support. The active ingredient, such as platinum and palladium, is impregnated into the support, such as carbon, silica and alumina. For the homogeneous catalyst system, an organometallic compound containing transition metal(s), such as nickel, cobalt and titanium, is mixed with an organometallic compound serving as a reductant, such as organo aluminum, organo magnesium and organo lithium. Compared to the heterogeneous catalysts, homogeneous catalysts are higher in hydrogenation activity and need a lower temperature and pressure for the catalytic reaction. In the case of polymers, the heterogeneous catalysts have more serious difficulty in hydrogenating them because of the high viscosity of the reaction system and the steric hindrance of the polymers, usually requiring severe reaction conditions. In particular, the selective hydrogenation of olefinic polymer in a copolymer containing conjugated diene monomer and vinyl aromatic monomer under such severe reaction conditions is extremely difficult, since the unsaturated double bonds of an aromatic compound could be simultaneously hydrogenated. Thus, the heterogeneous catalyst is economically unfavorable not only because a large quantity of the catalyst is required for effective hydrogenation, but also because of the high temperature and pressure causing the polymers to be decomposed and gelled.
In contrast, the homogeneous catalysts are very advantageous by virtue of high reactivity and hydrogenation efficiency even at a milder reaction such as a lower temperature and pressure.
Several methods to selectively hydrogenate the unsaturated double bonds of conjugated diene polymers have been disclosed as set forth hereunder. U.S. Pat. Nos. 3,644,588, 3,868,354, 3,541,064 and 3,700,633 disclose catalyst systems for hydrogenating or selectively hydrogenating ethylenically unsaturated polymers or ethylenically unsaturated aromatic copolymers, in which the metals of group VIII on the Periodic Table, especially, nickel or cobalt, are combined with the metals of groups IA, IIA and IIIB on the Periodic Table, especially, lithium, magnesium and aluminum alkyl as reductants. Such catalyst systems can effectively hydrogenate the unsaturated double bonds of the butadiene copolymer.
However, the above-described homogeneous catalysts have recognized some disadvantages in that a) since it is in general extremely sensitive to the outer circumstances, the catalyst may be easily decomposed in air, or in the presence of moisture, and b) the hydrogenation activity may greatly vary depending upon the reduction state of catalyst. In consequence, it is very difficult to satisfy the high degree of hydrogenation yield and reproducibility simultaneously. Such trend will badly affect the hydrogenation of a polymer designed to improve the durability and oxidation resistance of a polymer. Furthermore, the hydrogenation rate of the homogeneous catalyst is much affected depending on its stability during the hydrogenation.
Now that the industrial application of a homogeneous catalyst in the hydrogenation of polymer has faced the above problems, there are strong needs for the development of a highly active hydrogenation catalyst with better stability and reproducibility.